I am a college student and Adobe offers a great discount if you are a student. Currently, I own CS3 standard and have been learning to use its multiple features. I have been looking at what CS4 has to offer but I am not sure if its worth it to upgrade.

I tried the trial version and ran into a couple things I didn't like (couldn't open more than one file at a time, didn't seem to have a lot of new stuff). Try the trial for a few days and see what you think before switching. I skipped CS2 for similar reasons before switching to CS3 when it came out.

There are numerous articles detailing the "new stuff" to CS4 however the ones that stand out to me:

Use of dedicated video cards to full potential (no more zooming in and out of multiples of 50% to get proper aliasing on photos)

Content Aware Scaling

Adjustment Layers now operate seperate from picture (meaning no more having to close the layer to refocus picture, or zoom photo)

Tabbed files

Uhm. Lots more stuff.

However like upgrading a camera body, you're really only going to get "functional" differences between the two versions. If you have the cash to upgrade - do so to make things "easier"; if not - CS3 is still boss.